1. Field of the Invention
The present invention relates to circuit board fabrication and more specifically, it relates to a method for fabricating three dimensional imprint tools.
2. Description of Related Art
Previous attempts by others in making imprint tools have suffered largely in two areas. Firstly, the process required two plating steps and the adhesion of the nickel layers was insufficient. Secondly, they lacked control of the height/depth and the cross-sectional areas of the features were not accurately reproduces in the thermoset laminate material. These are both serious issues affecting the replication and functionality of the final product.
One method of making an imprint tool included etching a copper substrate to make a master. A pattern is chemically milled into the copper sheet and a nickel tool is electroformed from the master. Problems with this method include lack of control of the depth of the etch over large areas and the typical rounding of the pattern sidewall from etching. These are both serious problems since etching deeper causes more rounding and greater distortion of the pattern.
A second method uses a two-step plating process to form the trace and stud/via. This process starts with patterning the traces on a nickel sheet and plating the traces with nickel. This is followed by applying another photoresist layer and patterning the studs. This method suffered from lack of adhesion between the nickel layers, trace/stud, as well as the substrate. It is difficult aligning the second pattern over the first plated layer and it was difficult controlling the height of the studs during the second plating.
It is desirable to provide methods for making an imprint tool that are superior to the methods described above in eliminating adhesion problems, in controlling the feature height and in providing superior trace and stud cross-sectional areas.
It is an object of the present invention to provide a process for fabricating three dimensional imprint tools.
It is another object of the invention to eliminate adhesion issues by electroforming the trace/stud as an integral component.
Another object of the invention is to provide a method for controlling feature height and for overcoming overplating or geometry current density issues.
Still another object of the invention is to provide a method for fabricating three dimensional imprint tools having trace and stud cross-sectional areas that are superior over trace and stud cross-sectional areas produced with etching or by the prior art two step process.
To maintain consistency in the fabricated tool, the present invention provides a method for fabricating a tool Master from which the embossing tool of the present invention are made. The present method allows robustness to be built into the tool Master by making it any desired thickness. The use of two layers of dry film photoresist is unique to this invention. This method provides a tool designer the opportunity to incorporate any combination of embossing features into the design of the tool, including, but not limited to, features as small as a few microns to as much as 50 micron or more. Furthermore, the present invention also affords the flexibility to design tools having features of varying geometry xe2x80x9cstackedxe2x80x9d one on top of another feature, thus providing for a multi-level imprinting tool.
The use of liquid photoresist can also be used in this method, as well as the utilization of dry film photoresist. The dry film photoresist is produced with a very controlled manufacturing process that affords control over the thickness or height of the features that are grown.
Imprint technology can be used in all sectors of government and industry currently using conventional printed circuit technology. The present invention is useful in high density circuit fabrication because the via or plated through-hole is made concurrently with the trace formation. This allows closer spacing of traces and eliminates the need for large angular rings (pads) around the vias. This imprint technology eliminates several wet processing steps used in current technology, resulting in increased production quantity and decreased production cost compared to existing imprint technology.
The instant invention, therefore, is a method for the fabrication of an imprint tool Master and a method for fabricating an embossing tool using the tool Master. The process begins with a sheet of stainless steel or titanium having a thickness of about 0.090 inches. Stainless steel and titanium, of course, are illustrative materials only. Other choices of materials and material sheet thicknesses are possible so long as the choice is electrically conductive and exhibits a reasonable stiffness-to-weight ratio. A dry photoresist film is first laminated onto the stainless steel or titanium substrate sheet. (Liquid photoresist can also be used in this method, as an alternate to the use of dry film photoresist; however, the dry film photoresist contributes to a very controlled manufacturing process that affords control over the thickness or height of the features that are grown.) A mask comprising a negative trace image of a desired pattern, a circuit pattern for example, is then placed on the dry film photoresist and the film exposed to light through the mask openings. The mask and the Mylar cover sheet are then removed and a second dry film photoresist is laminated onto the first dry photoresist film if a multi-layer structure is desired. In such cases, a second mask comprising a second negative trace image is aligned over the second dry film photoresist and the second film exposed as was the first.
The next step in this process is to use standard developing methods to develop both of the exposed dry film photoresist layers at the same time. This results in the removal of the unexposed portions of the dry film photoresist layers, leaving only the exposed portions on the stainless steel or titanium substrate sheet. The surface of this sheet and the developed portions of the photoresist are then covered with a copper film in order to form a conductive adhesion layer and a thick plate of nickel electroplated onto the copper layer. This nickel layer is typically about 0.030 inches to about 0.040 inches thick but thicker or thinner layers are possible, and may be desirable, given a particular end-use circumstance.
In a final step, the electroplated thick nickel layer is removed from the stainless steel or titanium sheet, is cleaned to remove copper and photoresist residue, and is chemically passivated for further processing.
The cleaned nickel plate now embodies a tool master, from which an embossing tools can be fabricated.